Marking a function as const: what does the compiler do?

[VoB]

Hi everyone, I'm trying to understand what happens when a function is marked as const in a class. Consider the following simple class, where a function foo is used as a test function.

#include <iostream>

class ptrclass
{
private:
    int *elem;

public:
    ptrclass(int i) : elem{new int[i]} {}

    void foo() const
    {
        elem[2] = 9;
    }

    ~ptrclass()
    {
        delete[] elem;
    }
};

int main()
{
    ptrclass p{10};
    p.foo();


    return 0;
}

Since foo() is marked as const, then I know that the compiler puts a const in front of int *elem;, BUT as you can see, the code compiles even if I modify what is pointed to by elem

Question: I'd conclude that the compiler does write

int *const a; (constant pointer to an integer)

instead of

const int* a; (pointer to a constant integer)

because I get no compiler error. Is this correct?

[Ganado]

Right, the 'const' here does NOT propagate through to the data that is being pointed to. The pointer itself is const, and it will only be a compiler error to change the pointer itself in a const member function.

There's a proposal for a class that does magic to allow const to propagate.
https://en.cppreference.com/w/cpp/experimental/propagate_const
I have not personally tried it out, though.

[VoB]

Thanks Ganado for your quick reply and check.

Actually, I was expecting const int* a; to be used.

Just one last quick question: is it correct to say that const int* a means that "a is a pointer to a constant integer"?

I know that the above syntax, in practice, implies that I can't change the data pointed to by a, i.e. I can't do something like a[2] = 3;. But this is a bit weird to me, because actually I have an array of constant integers!

Please tell me what is the correct way in your opinion

[Ganado]

const int* a means that "a is a pointer to a constant integer"?

Correct. Sometimes, it's more helpful to read the syntax right-to-left: "pointer to int that is const".

Then, int *const could be read as, "a const pointer to an int".

and const int * const would be "const pointer to a const int"
(same thing as int const * const)